With the ever-increasing popularity and competitiveness of golf, substantial effort and resources are currently being expended to improve golf clubs so that increasingly more golfers can have more enjoyment and more success at playing golf. Much of this improvement activity has been in the realms of sophisticated materials and club-head engineering. For example, modern “wood-type” golf clubs (notably, “drivers” and “utility clubs”), with their sophisticated shafts and non-wooden club-heads, bear little resemblance to the “wood” drivers, low-loft long-irons, and higher numbered fairway woods used years ago. These modem wood-type clubs are generally called “metal-woods.”
An exemplary metal-wood golf club such as a fairway wood or driver typically includes a hollow shaft having a lower end to which the club-head is attached. Most modem versions of these club-heads are made, at least in part, of a light-weight but strong metal such as titanium alloy. The club-head comprises a body to which a strike plate (also called a face plate) is attached or integrally formed. The strike plate defines a front surface or strike face that actually contacts the golf ball.
The current ability to fashion metal-wood club-heads of strong, light-weight metals and other materials has allowed the club-heads to be made hollow. Use of light-weight materials has also allowed club-head walls to be made thinner, which has allowed increases in club-head size, compared to earlier club-heads. Larger club-heads tend to provide a larger “sweet spot” on the strike plate and to have higher club-head inertia, thereby making the club-heads more “forgiving” than smaller club-heads.
The distribution of mass around the club-head typically is quantified by parameters such as rotational moment of inertia (MOI) and CG. Club-heads typically have multiple rotational MOIs, each associated with a respective Cartesian reference axis (x,y,z) of the club-head. A rotational MOI is a measure of the club-head's resistance to angular acceleration (twisting or rotation) about the respective reference axis. The rotational MOIs are related to, inter alia, the distribution of mass in the club-head with respect to the respective reference axes. Each of the rotational MOIs desirably is maximized as much as practicable to provide the club-head with more forgiveness.
Regarding the total mass of the club-head as the club-head's mass budget, at least some of the mass budget must be dedicated to providing adequate strength and structural support for the club-head. This is termed “structural” mass. Any mass remaining in the budget is called “discretionary” or “performance” mass, which can be distributed within the club-head to address performance issues, for example.
As noted above, an important strategy for obtaining more discretionary mass is to reduce the wall thickness of the club-head. For a typical titanium-alloy “metal-wood” club-head having a volume of 460 cm3 (i.e., a driver) and a crown area of 100 cm2, the thickness of the crown is typically about 0.8 mm, and the mass of the crown is about 36 g. Thus, reducing the wall thickness by 0.2 mm (e.g., from 1 mm to 0.8 mm) can yield a discretionary mass “savings” of 9.0 g.
Modern hollow metal club-heads, particularly of the “metal-wood” type, are made by investment casting, which is the best known method for forming the intricate surficial and interior details of the club-head at a practical cost. In investment casting, reducing club-head wall thickness, however, is not easily achieved. Forming a thinner wall requires a correspondingly narrower mold cavity to which greater force must be applied to urge molten metal fully and completely into the cavity. Also, narrower mold cavities and higher pressures increase the probability that the metal will flow turbulently into the cavities, wherein turbulent flow tends to generate casting defects. Other engineering challenges include achieving the desired strength and surface requirement of the cast part, achieving the desired combination of high yield and low material usage (two conflicting requirements), and using revert to further reduce costs.